The invention relates in general to power demand control for switching loads according to a priority schedule in a plant facility supplied with energy from a utility company so as to keep the demand at the end of a period of control within a preassigned demand limit. The invention relates in particular to a power demand control system in which the priority schedule is changed or fixed in time in accordance with the dynamics of operation of at least one of the loads to be switched, or in accordance with operator initiated commands. Such adaptive load priority determination is particularly applicable to arc furnace installations, since in such installations the decision to switch, or not to switch, an arc furnace should not only take into account the requirements of power demand limit but also the economics and requirements of production.
It is known from U.S. Pat. No. 3,602,703 of T. R. Polenz to switch a load of highest priority and control the power available to be switched after such switching by considering the known pattern of additional loads, so that so much of the successive stages of such an additional load can be switched as the remaining available power within an assigned demand limit. Typically, such loads can be individually controlled so as to stretch or contract the known pattern of energy consumption.
It is known from U.S. Pat. No. 3,714,453 of J. Delisle et al to control switching of a plurality of loads on the basis of the power consumed by the loads and of the demand derived from the utility company.
The prior art of power demand control has been described and a technique used has been disclosed in U.S. Pat. No. 3,872,286 of R. Putman issued on Mar. 18, 1975. For the purpose of the description hereinafter of the preferred embodiment of the present invention, the description and drawings of the Putman patent are hereby incorporated by reference. The general technique of power demand control consists in predicting at successive operative points during a demand period, of for instance 15 minutes, the expected demand at the end of the period so as to derive an error (by reference to a demand limit not to be exceeded). Shedding (switching OFF) or restoring (switching ON) of loads according to a priority table is performed by a computer on the basis of a computation of such demand error, which can be positive or negative. The Putman patent discloses a more refined controlling approach by the calculation of a residual error after calculation of a predicted result due to control of selectable loads, such residual error being used in subsequent calculations to determine if additional loads should be manipulated.
While very effective in adjusting the power demand during each demand period within the limit of an assigned target, the control technique of the Putman patent has been mainly applied in the past to derive substantial savings from the control of a category of loads having a controllable status as opposed to other loads left totally out of control, the latter because of constraints imposed on the loads, or because production requirements dictate that these other loads remain unaffected by power demand control. This all or nothing approach with respect to control of the main units of production reduces the overall efficiency of power demand control. If indeed substantial savings can be derived from controlling under a priority schedule many loads which are supporting the production of a user's plant, it is desirable to include the main production units among the loads having a controllable status from a point of view of power demand control.
An object of the present invention is to provide a power demand control system which includes besides its sheddable and reconnectable loads, loads which are adjustable production units. Typically, these will be a relatively few loads of relatively large size, as opposed to numerous, small loads.
Another object of the present invention is to provide, in accordance with a priority schedule, switching or adjustment of loads including production units in order to more closely meet a power demand limit not to be exceeded at the end of any of successive demand periods.
A further object of the present invention is to combine scheduling of sheddable and reconnectable loads for limiting the demand of power in a user's facility with scheduling for adjustable production loads, the load selection being in accordance with the production process within the facility.
Still another object of the present invention is to determine with a computer the initiation and the termination, and/or the level of consumption of a plurality of production processes consuming energy in order to maximize production under the constraint of an assigned power demand limit.